I'm running a Navien tankless in the garage at the moment. I sometimes hear a vibration as the burner and the fan kick in. It's not's much, but it's different than the old gas water heater tank that didn't use a fan.
We installed this tankless unit about 2.5 months ago and have been having trouble with a deep rumble sound that reverberates throughout the house. It only occurs at random times and lasts only a couple of seconds. We have 3" pipe - exhaust and intake next to each other going horizontal from the unit and then vertical in the existing 8" flue going up about 15' through the roof. On the roof the exhaust pipe goes straight up with a cap on it, while the intake is 3' lower with an elbow pointing sideways. We live at about 6500' and have adjusted the dip switches for altitude. Still getting these random rumbles. Does anyone have other ideas about what else we can try to eliminate this problem.
My rumble is a bit more of a problem. It is quite loud throughout the house even though the unit is in the crawl space. It definitely sounds - not right....
Is this house is on a well? Most tankless units require a minimum water pressure on the order of 15-20psi to keep the microboil on the heat exchanger from going into macro steam bubbles. When that occurs it will have any thing from an audible sizzle to some pretty significant rumble & bang.
If you are on a well system with a bladder-tank reservoir set up to not kick the well pump on until the pressure is 12psi or something, when you run the hot water when the tank is at the lower end of it's pressure range you could have a fairly random occurrence of some pretty noisy stuff.
(Just one less-obvious possibility to consider.)